Working 13 rounds means you start counting to 13 from this point forward. When you finish your 13 rounds, you would have a total of 18 -- 5 original rounds + 13 just worked.

The term "even", does, indeed mean working with no additional increases or decreases as Wenda explained. However, it also means continuing in whatever stitch pattern you have established. For example, if your first 5 rows were in st st, then your 13 rounds would also be st st. If your first 5 rows were seed stitch or a rib pattern, then your 13 rounds would be worked in that same seed stitch or rib pattern.